1. Field of the Invention
The present invention relates to gas turbine engines, and more particularly, to burners for combustors in gas turbine engines.
2. Description of Related Art
A variety of devices and methods are known in the art for injecting fuel into gas turbine engines. Of such devices, many are directed to injecting fuel into combustors of gas turbine engines to sustain combustion under lean conditions. Lean combustion is desirable for low power settings in gas turbine engines because it is fuel efficient and can produce relatively low levels of undesirable emissions. Aero gas turbine engines have progressively been designed to operate leaner and leaner in order to reduce NOX emissions. Industrial gas turbine engines have been switching to lean partially premixed combustion to operate lean, primarily to lower NOX production rates. However, the trend toward lean combustion has been impeded by operability concerns. Very lean combustion has proven to be very unstable. Flames produced in lean conditions tend to be unstable and if left unchecked the instability can result in lean flame blow out. Moreover, even if lean blow out does not occur, instabilities in lean combustion can result in strong acoustic waves that can cause undesirable noise and stress within the structures of a gas turbine engine. Measures can be taken to mitigate instabilities and control the combustion process to improve flame stability. However, at very lean conditions, e.g., below around 0.60 equivalence ratio for a gas turbine using liquid fuel, conventional methods may not be enough to provide the desired stability.
One way of providing stable combustion at very lean conditions is to use a flameless combustion process. Most combustion instabilities involve a three part cyclic process, where fluid mechanical phenomena result in a fluctuation in heat release rate that couples and reinforces an acoustic mode, which in turn trips an unstable fluid dynamic structure, which leads to fluctuations in heat release rate, and so on. In flameless combustion, such a coupling does not occur. The inability of coupling of this kind to occur in flameless combustion inhibits strong acoustic waves that could otherwise damage the combustor or turbine blades.
Flameless combustion has been successfully demonstrated in industrial furnaces. The technique involves using a very lean mixture wherein high temperature oxidizer reacts with fuel at very high levels of turbulence in a distributed reaction zone. Flameless combustion has been shown to produce very stable combustion having low NOX levels in industrial furnaces. This combustion method is called “flameless combustion” because of the lack of a discrete visible flame resulting from the distributed nature of the reaction. In industrial furnace applications of flameless combustion, the high oxidizer temperatures required are obtained by either preheating the air with furnace exhaust gases through a heat exchanger or by direct mixing of the air with hot recirculated exhaust gas. These furnaces typically recycle combustion gases via a duct external to the combustion region. These ducts and/or heat exchangers in conventional burners have limited the application of flameless combustion to ground based operations.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, there is still a need for improved devices and methods for producing flameless combustion without recirculation ducts or heat exchangers. The present invention provides a solution for these problems.